A flow sensor having a sensor unit disposed in an air flow is disclosed in U.S. Pat. No. 4,501,144. A heater and two temperature sensors are arranged on the sensor unit. Of the two temperature sensors, one is disposed forward of the heater when viewed in the direction of flow and the other sensor is disposed rearward of the heater. The two temperature sensors are arranged in a measuring bridge and the measuring signal is formed from a comparison of the resistance values. The temperature of the heater is controlled in such a manner that the overtemperature, that is, the temperature difference between heater and air flow, is constant. A constant overtemperature has, however, the disadvantage that the temperature dependency of the sensor characteristic cannot be completely compensated.
U.S. Pat. No. 4,777,820 discloses a flow sensor wherein the overtemperature is a function of the air temperature in order to compensate for the temperature dependency of the sensor characteristic. The measurement of the sensor signal takes place by means of the measurement of the voltage of the heater resistor. The correction of the temperature dependency of the characteristic of the sensor is limited because of this measuring principle.